The present invention is directed to heating, ventilating and air conditioning (HVAC) systems, to refrigeration systems, and to chiller systems which modulate an expansion valve to maintain a system condition such as superheat, refrigerant liquid level, or chilled water temperature. The present invention proposes to also modulate the expansion valve to maintain minimum lubricant flow to the compressor or compressors. For purposes of this application, chiller systems is defined to also include HVAC systems and refrigeration systems.
Certain systems use the differential pressure across the compressor to return lubricant to the compressor. The lubricant is used in the compressor to lubricate bearings or the like and to seal the gap between the compressor's rotors, wraps or other compressing elements.
In some systems, the expansion valve is modulated to maintain refrigerant liquid level control in one of the system heat exchangers. The condensing heat exchanger can be cooled by a chilled water loop provided by, for example, a cooling tower and determined by a cooling water temperature. The evaporating heat exchanger can provide chilled water for use as a heat transfer medium and the expansion valve can be modulated to maintain the chilled water temperature of the fluid provided by the evaporating heat exchanger. If the evaporating heat exchanger is a falling film type evaporator, the expansion valve is modulated to maintain a liquid level in the evaporating heat exchanger.
With such liquid level control, the differential pressure across the compressor is determined by the difference between the cooling water temperature and the chilled water temperature. If the difference between the cooling water temperature and the chilled water temperature is small or inverted, the differential pressure will be too small to pump lubricant back to the compressor. The chiller system will shutdown on a low oil flow diagnostic or a loss of oil diagnostic. The conditions causing this are typical of those which occur when a system is started with a low cooling tower temperature and warm chilled water temperature.
More specifically, under normal running conditions, the liquid level controller maintains a pool of liquid in the bottom of the evaporating heat exchanger. A liquid level sensor measures the depth of the pool and a PID algorithm in the controller maintains a desired level by modulating an electronic expansion valve to change its position and affect the rate of refrigerant flow into the evaporator. The liquid level controller maintains a mass balance between the flow of refrigerant vapor removed from the evaporator by the compressor and the flow of liquid refrigerant returned from the condenser to the electronic expansion valve. When the electronic expansion valve is opened, the flow of refrigerant into the evaporator increases and at some point will exceed the flow out of the evaporator. This causes the condenser to drain to the point that the vapor will flow from the condenser to the evaporator rather than liquid refrigerant. Mass balance will then be re-established because of the refrigerant vapors lower density. However, the flow of refrigerant vapor reduces the chiller system efficiency because the vapor is eventually pumped back to the condenser without providing effective cooling.
On the other hand, when the expansion valve is closed, refrigerant flow out of the evaporator is such that it is less than the flow in. This causes the evaporator pool to fall and eventually dry out. Because the compressor is removing more refrigerant from the evaporator than the electronic expansion valve is allowing to enter the evaporator, the evaporator pressure will fall. As this evaporator pressure falls, the differential pressure across the compressor increases. The higher differential pressure reduces the compressor efficiency and flow through the compressor falls such that the mass flow balance is re-established but the chiller efficiency is again reduced.
It would be advantageous that the expansion valve could be controlled to both maintain the liquid level and to maintain the compressor pressure differential at or above a desired minimum threshold.